Blending short layered hair with long extensions

I got my sister to install 18inch light brown microring hair for me. I have short hair, layered, dark brown underneath and blonde streaked/light brown on top. I dont think she put enough hair in because you could still see my short dark hair underneath, and on top, my own layered hair stopped just above ear length, and just sort of sat there on top of the extensions like a silly looking "cap". How on earth do I get them to blend in? You could really tell where my own hair was and the extensions began. I took the extensions out and am going to try again, but I'm going to get dark brown hair for underneath, to disguise my own short dark hair, and the same light brown on top because I dont like my hair too dark, but I dont want to waste more money. Help!

I didn't even take a picture of the hair extensions before I took them out, but I can post a picture of my short hair without the extensions so you know what I have to work with. It looked great from the front, but from the back it was a trainwreck! I had 125 extensions in altogether.  Might need 200 I reckon.  Also how do you wash your hair with them in? I was too scared in case they fell out! 

How short is your hair?
With shorter hair, it is important to both use a lot of hair (especially since you want to use 18" long extensions hair) and to get your bond or rows high enough. To blend well, you will probably have to use at least 6 oz. of hair, and have rows up very close to your crown and top part in yoru hair. As high as you can go.
The shorter your hair is and the longer you try to get the length to be, the more hair you will need to use and the higher up the bonds will need to be in order to blend well.
Layering does also help. slight graduated layers can do wonders for blending.  Also- styling, be it with a flat iron or curls, can help as well.
It sounds to me like you may need more hair and higher rows.

Oh that's not so short. You can make it work.
Try a bit more hair, and higher rows. As high as you can go without them being really detectable.
Another option for you is to try the weft methods. While strand-by-strand can give you good coverage, it is often easier to get more full coverage with wefts. Or you can mix the two. In my experience, weft methods (be it adhesive tapes, bonding with Liquid Gold, clips ins, sew-ins, etc) have provided fuller coverage and made for easier blending when my hair was shorter.
Good luck! I know you will be able to get them to work well. Then you'll be addicted!